A typical exhaust purifying apparatus applied to an internal combustion engine such as a vehicle diesel engine includes a PM filter (refer to Japanese Laid-Open Patent Publication No. 2002-227688) that is located in an exhaust system. The PM filter traps particulate matter (“PM”), which is predominantly composed of soot in exhaust gas. In an internal combustion engine provided with such an exhaust purifying apparatus, PM elimination control is performed to prevent the PM filter from being clogged with particulate matter. In the PM elimination control, particulate matter is burned and removed.
In the PM elimination control disclosed in Japanese Laid-Open Patent Publication No. 2002-227688, the exhaust air-fuel ratio is reversed between a rich state and a lean state to supply an unburned fuel component and oxygen to a catalyst of the PM filter. Oxidation of the unburned fuel component is used to increase the catalyst bed temperature and to burn particulate matter in the PM filter. The supply of unburned fuel component to the catalyst in the PM elimination control is performed in a manner that enables a temperature increase in the catalyst with a minimized amount of unburned fuel. This is to minimize extra fuel consumption due to the PM elimination control.
However, it has been confirmed that, in the above PM elimination control, not all particulate matter deposited at the front end of the PM filter (upstream end relative to the flow direction of exhaust gas) can be burned. The reasons for this are considered to be as follows.
The front end of a PM filter is susceptible to deposition of particulate matter, and the amount of deposited particulate matter is more than that in the downstream section.
The PM elimination control, in which unburned fuel component and oxygen are supplied to the PM filter, cannot supply a sufficient amount per unit time of unburned fuel component and oxygen to burn all the particulate matter deposited at the front end of the PM filter.
Accordingly, it is inferred that, even if the PM elimination control is performed for an extended time, unburned PM remains at the front end of the PM filter. If the exhaust air-fuel ratio is richened after the PM elimination control is ended with some remaining PM, unburned fuel component is supplied to the PM filter, and oxidation of the unburned fuel causes the particulate matter to burn, which can excessively increase the catalyst bed temperature of the PM filter.